Exposure apparatus and method of cleaning optical element of the same

ABSTRACT

An exposure apparatus for exposing a substrate to a pattern of a mask by use of exposure light. The apparatus includes an optical system for directing the exposure light from a light source to the substrate, the optical system having an optical element, a first casing for accommodating therein an optical surface of the optical element, and a second casing for accommodating therein the optical element and the first casing, a first port provided in the first casing, a second port provided in the second casing, a supplier for supplying an inert gas into the first casing and the second casing, and a supplier for supplying an inert gas into the first casing and the second casing through the first port and the second port.

FIELD OF THE INVENTION AND RELATED ART

This invention relates generally to an exposure apparatus which uses, asexposure beam, short wavelength electromagnetic waves such as X-rays orultraviolet rays from an excimer laser, for example. More specifically,the invention is concerned with such exposure apparatus, a devicemanufacturing method using the same, and a method of clearing an opticalelement of such exposure apparatus.

In projection exposure apparatuses for manufacture of semiconductorintegrated circuits, light of various wavelength bands is projected asexposure beam to a substrate. As for such exposure beam, examples aree-line (wavelength λ=546 nm), g-line (λ=436 nm), h-line (λ=405 nm),i-line (λ=365 nm), KrF excimer laser (λ=248 nm), ArF excimer laser(λ=193 nm), and X rays.

An exposure beam emitted from a light source is directed by way of anillumination optical system for illuminating a reticle and a projectionoptical system (projection lens) for imaging a fine pattern formed onthe reticle upon a photosensitive substrate, whereby the fine pattern islithographically transferred to the photosensitive substrate. In suchexposure apparatuses, miniaturization of pattern linewidth has forcedfurther improvements of throughput and resolution. Also, an exposurebeam of higher power has been required and, on the other hand,shortening of wavelength band of exposure beam has been required.

It is known that when an exposure beam of i-line (wavelength λ=365 nm)or a shorter wavelength is used, due to shortening of the wavelength,impurities in the air photochemically may react with oxygen. The product(blurring material) of such reaction may be deposited on an opticalelement (lens or mirror) of the optical system, causing non-transparent“blur”.

As regards such blurring material, in a case where sulfurous acid SO₂absorbs light energy and it is excited thereby, a typical example may beammonium sulfate (NH₄)₂SO₄ produced by reaction (oxidization) withoxygen in the air. When such ammonium sulfate is deposited on thesurface of an optical element such as a lens or mirror, theabove-described “blur” results. Then, the exposure beam is scattered orabsorbed by ammonium sulfate, so that the transmission factor of theoptical system decreases. This causes a large decrease of light quantity(transmission factor) upon the photosensitive substrate, and thus adecrease of throughput.

Particularly, for ArF excimer laser (193 nm) or X-rays which are in avery short wavelength region, the exposure beam may cause strongphotochemical reaction. Thus, the above-described problems is veryserious.

SUMMARY OF THE INVENTION

Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994 showsan arrangement wherein a barrel having mounted therein a glass membersuch as a lens is provided in a casing of closed structure, and whereinthe inside of the casing is filled with an inert gas, thereby to solvethe problem such as described above.

However, it has been found that, in such example using inert gas, anoptical element within the barrel or casing of the illumination opticalsystem may be contaminated by organic molecules. These molecules may bethose of any solvent, for example, used during manufacturing and workingprocesses of components of the illumination optical system and remainingon the components, or those of adhesive agent used in the casing orbarrel and evaporated therefrom.

Taking the manufacturing procedure into consideration, the environmentalair may be contaminated by organic molecules coming from an adhesiveagent layer between a substrate and a photoresist, for example. Thesemolecules may enter the casing or barrel. Even if the organic moleculesare at a low concentration, particles may be decomposed due to theinfluence of ultraviolet beam and they may be deposited on the opticalelement. In that occasion, a carbon film or a film containing carbonwill be produced on the element surface.

Japanese Laid-Open Patent Application, Laid-Open No. 209569/1995 showsan arrangement wherein, when an inert gas is supplied into a projectionoptical system, a small amount of ozone is mixed into the inert gas,such that an inert gas containing ozone is supplied to an opticalsystem. The optical element is irradiated with an exposure beam in a gasambience containing ozone and, due to ozone cleaning effect,decomposition of organic molecules on the surface of the optical elementas well as deposition of product of decomposition thereon are prevented.

In this structure, however, an ozone generator having a Hg lamp isprovided in a portion of an inert gas supplying line. The ozonegenerator produces ozone beforehand, and then the ozone is supplied intothe lens holder. This structure needs use of two light sources, that is,the exposure light source and the ozone generating light source. Thestructure is thus complicated. Further, this creates the followingdangerous possibilities. That is, ozone has a property for deterioratingan element. Therefore, the ozone generator itself may be easily damagedby the influence of ozone. Thus, there is a possibility of leakage ofharmful ozone from the damaged ozone generator.

It is an object of the present invention to provide an exposureapparatus by which contamination of an optical element by organicmolecules can be prevented, particularly, very simply and effectively.

It is another object of the present invention to provide a devicemanufacturing method using such exposure apparatus and/or a method ofcleaning an optical element of an exposure apparatus.

In accordance with an aspect of the present invention, there is providedan exposure apparatus, comprising: a light source for producing anexposure beam; an optical system having a closed space, for projectingthe exposure beam to a substrate for exposure thereof; first supplyingmeans for supplying an inert gas into the closed space of said opticalsystem; and second supplying means for supplying one of oxygen and aclean air, so that the inert gas and oxygen can be supplied to theclosed space.

In accordance with another aspect of the present invention, there isprovided a device manufacturing method comprising the steps of:preparing an exposure apparatus as recited above; and performing anexposure process by use of the exposure apparatus.

In accordance with a further aspect of the present invention, there isprovided a method of cleaning an optical element of an exposureapparatus for exposing a substrate with an exposure beam of ultravioletrays or X-rays, projected thereto, said method comprising the steps of:supplying an inert gas containing a small amount of oxygen into a spacein which the optical element is placed; projecting the exposure beam sothat ozone is produced in the space; and removing an organic compounddeposited on the optical element through photochemical reaction byprojection of the exposure beam and the produced ozone.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a general structure of an exposureapparatus according to an embodiment of the present invention.

FIG. 2 is a sectional view for explaining an example of inside structureof a barrel.

FIG. 3 is a flow chart of device manufacturing processes.

FIG. 4 is a flow chart for explaining details of a wafer process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings. In an embodiment of thepresent invention to be described below, the invention is applied to anexposure apparatus of reduction projection type, which is generallycalled a stepper or a scanner.

The exposure apparatus Al has a major assembly which may be roughlyseparated into a light source 1 (ArF excimer laser light source), alight source lens system 2 which comprises an illumination opticalsystem for transforming laser light L1, which is illumination lightemitted from the light source 1, into light of a predetermined shape,and a projection lens system 5 for imaging the laser light L1, havingbeen formed into a predetermined shape by the light source lens system2, upon a wafer 4 (substrate) through a reticle 3. The light source 1includes a laser control device 6 for controlling laser output of thesame. The laser control device 6 is controlled by a controller 7(control means). The laser control device 6 functions to change theemission laser wavelength region, to be described later.

In this embodiment, the light source comprises ArF excimer laser whichproduces ultraviolet rays. However, it may comprise a KrF excimer laserlight source or, alternatively, an X-ray source for producing shorterwavelength X-rays (X-rays are referred generally to soft X-rays orvacuum ultraviolet rays, for example), such as a synchrotron radiationsource or a laser plasma radiation source, for example.

The light source lens system 2 comprises barrels 2 h and 2 i in which aplurality of lenses 2 a-2 d are disposed therewithin. These barrels areaccommodated in a casing 2 g. Disposed on the opposite sides of them aremirrors 4 a and 4 b for deflecting and directing the laser light along apath. The casing 2 g of the illumination optical system has a window 2 edemountably mounted at an entrance opposed to the light source L andalso a window 2 f at an exit opposed to a reticle 3.

The projection optical system 5 has a plurality of lenses 5 a and 5 baccommodated in a lens barrel, for projecting a pattern of the reticleonto a wafer in a reduced scale. There are windows 5 c and 5 d mountedon the faces opposed to the reticle and the wafer, respectively.

Inert gas of nitrogen gas (it may be a gas of helium or neon, forexample) is supplied to an inside space 2 j of the casing 2 g of theillumination optical system 2, to the inside spaces of the barrels 2 hand 2 i, and to the spaces inside the projection lens 5 and the lensbarrel as separated by lenses. Inert gas supplying device 8 a isconnected to these spaces through an inert gas supplying line 8 b and anelectromagnetic valve 8 c (opening/closing valve) provided in a portionof this line. At an intermediate position on the inert gas supplyingline 8 b, there is an oxygen supplying line 10 b connected as a branch.Through an electromagnetic valve 10 c (opening/closing valve) providedin a portion of this line, an oxygen supplying device 110 a is connectedthereto. This enables mixture of a small amount of oxygen into the inertgas to be supplied. In place of pure oxygen, a clean air which containsoxygen may be mixed.

There is a gas discharging device 9 a for discharging gases from thesespaces to which inert gas is supplied. These spaces are connected to thegas discharging device 9 a through a gas discharging line 9 b and anelectromagnetic valve 9 c (opening/closing valve) provided in a portionof this line.

The gas collected by the gas discharging means may include a smallamount of residual ozone. The gas discharging device 9 a may be providedwith converter means for re-converting the residual ozone into oxygen.Impurities may be removed by a filter and thereafter the oxygen thusre-converted may be circulated to the oxygen supplying device 10 a, forreuse thereof.

The electromagnetic valves 8 c and 9 c provided in portions of the linefor the inert gas supplying means and for the gas discharging means arecontrolled in accordance with a program set in the controller 7beforehand, so that the spaces are kept in a state as filled with inertgas, during normal operation of the apparatus (exposure process) or astand-by period.

More specifically, in accordance with preset timing, in the stand-bystate of the apparatus, the electromagnetic valve 10 c of the oxygensupplying line opens so that a small amount of oxygen is mixed into thenitrogen gas. The resultant gas is supplied into the casing and barrelsof the illumination optical system and also into the barrel of theprojection optical system. The opening/closing of the electromagneticvalve is controlled so that the amount of oxygen supply is kept notgreater than a predetermined concentration (e.g., not greater than a afew grams per 1 m³). After the mixture gas is supplied, theelectromagnetic valves 8 c and 9 c are closed. In the state in which thespaces are filled with a gas in which a small amount of oxygen is mixedinto a nitrogen gas, projection of laser light is performed. In responseto this, in these spaces, oxygen within the inert gas, filling thespace, is converted into ozone through photochemical reaction. Thus, inthese spaces, ozone is produced first. Laser projection is continued inthis state and, as a result, any organic compound deposited on opticalelements (lenses, mirrors or windows) constituting the optical system isoxidized. Consequently, organic molecules on the optical element isremoved by ozone cleaning, whereby the optical element is cleaned.

Subsequently, the electromagnetic valves on the inert gas supplying lineand the gas discharging line are opened, and supply of inert gas anddischarging of the gas are continued interruptedly or uninterruptedlyuntil the inside gas is completely replaced by nitrogen gas. The seriesof these sequential operations are performed in accordance with aprogram set in the controller beforehand. As regards the cleaning ofoptical elements, it may preferably be made during the stand-by periodof the apparatus in which the apparatus is held inoperative, since itdoes not influence the throughput. Alternatively, cleaning may be madeduring actual operation of the apparatus.

The efficiency of producing ozone from oxygen by light projectionlargely depends on the wavelength of light projected. In considerationof this, in order to assure efficient production of ozone, thewavelength of exposure beam may preferably be changed, between theexposure process for a substrate and the cleaning operation for opticalelements. More specifically, for the cleaning operation, preferably thewavelength region may be oscillated continuously or it may be changed toshorter wavelength side, by which the ozone production efficiency andthus the cleaning capacity can be improved. Changing the wavelength maybe accomplished by controlling the light source actuation or byinserting wavelength changing means (such as a harmonic wave producingelement, for example) into the light path, for example.

FIG. 2 is an enlarged view of a portion of the barrel 2 h of theillumination optical system, in the neighbourhood of a gas blowing portof the inert gas supplying line. As regards optical elements 13 a and 13b, it is expected that a large amount of organic molecules may bedeposited on these elements. Thus, the structure is so arranged that theinert gas is directly blown against these optical elements. Thisenhances the ozone cleaning effect considerably.

The inert gas supplying line may be provided with flow rate changingmeans such as disclosed in Japanese Laid-Open Patent Application,Laid-Open No. 216000/1994, so that nitrogen can be supplied at a largeflow rate for the impurity removing operation.

Exposure apparatus may include optical elements other than lenses,mirrors or windows as described. An example is a filter for transmittingonly a desired wavelength of light among a broad wavelength band asemitted by a light source such as Hg lamp or synchrotron radiationsource. The advantageous effects of the present invention describedabove similarly applies to such optical element.

Next, an embodiment of a device manufacturing method which uses anexposure apparatus such as described above, will be explained.

FIG. 9 is a flow chart of procedure for manufacture of microdevices suchas semiconductor chips (e.g. ICs or LSIs), liquid crystal panels, CCDs,thin film magnetic heads or micro-machines, for example.

Step 1 is a design process for designing a circuit of a semiconductordevice. Step 2 is a process for making a mask on the basis of thecircuit pattern design. Step 3 is a process for preparing a wafer byusing a material such as silicon. Step 4 is a wafer process which iscalled a pre-process wherein, by using the so prepared mask and wafer,circuits are practically formed on the wafer through lithography. Step 5subsequent to this is an assembling step which is called a post-processwherein the wafer having been processed by step 4 is formed intosemiconductor chips. This step includes assembling (dicing and bonding)process and packaging (chip sealing) process. Step 6 is an inspectionstep wherein operation check, durability check and so on for thesemiconductor devices provided by step 5, are carried out. With theseprocesses, semiconductor devices are completed and they are shipped(step 7).

FIG. 10 is a flow chart showing details of the wafer process.

Step 11 is an oxidation process for oxidizing the surface of a wafer.Step 12 is a CVD process for forming an insulating film on the wafersurface. Step 13 is an electrode forming process for forming electrodesupon the wafer by vapor deposition. Step 14 is an ion implanting processfor implanting ions to the wafer. Step 15 is a resist process forapplying a resist (photosensitive material) to the wafer. Step 16 is anexposure process for printing, by exposure, the circuit pattern of themask on the wafer through the exposure apparatus described above. Step17 is a developing process for developing the exposed wafer. Step 18 isan etching process for removing portions other than the developed resistimage. Step 19 is a resist separation process for separating the resistmaterial remaining on the wafer after being subjected to the etchingprocess. By repeating these processes, circuit patterns are superposedlyformed on the wafer.

With these processes, high density microdevices can be manufactured.

In accordance with an exposure apparatus of the present invention, theproblem of deposition of organic molecules on an optical element andreduction of illuminance thereby can be removed. Since the exposure beamitself to be used for the exposure process is used also for ozoneproduction, use of additional and large mechanism is not necessary.Further, since ozone production is made only inside a closed space inwhich the optical element is disposed, there is no dangerous possibilityof leakage of harmful ozone.

Since the exposure apparatus provides high throughput constantly, a highproductivity is accomplished in the device manufacture.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

1-18. (canceled)
 19. An exposure apparatus for exposing a substrate to apattern of a mask by use of exposure light, said apparatus comprising:an optical system for directing the exposure light from a light sourceto the substrate, said optical system having an optical element, a firstcasing for accommodating therein an optical surface of said opticalelement, and a second casing for accommodating therein said opticalelement and said first casing; a first port provided in said firstcasing; a second port provided in said second casing; and a supplier forsupplying an inert gas into said first casing and said second casing;and a supplier for supplying an inert gas into said first casing andsaid second casing through said first port and said second port.
 20. Anapparatus according to claim 19, further comprising (i) a third portprovided in said first casing, (ii) a fourth port provided in saidsecond casing, and (iii) an exhauster for discharging a gas in saidfirst casing and said second casing through said third port and saidfourth port.
 21. A method of manufacturing a device, said methodcomprising steps of: exposing a substrate to a pattern of a mask by useof an exposure apparatus as recited in claim 19; developing the exposedsubstrate; and processing the developed substrate to produce the device.22. An exposure apparatus for exposing a substrate to a pattern of amask by use of exposure light, said apparatus comprising: an opticalsystem for directing the exposure light from a light source to thesubstrate, said optical system having an optical element, a first casingfor accommodating therein an optical surface of said optical element,and a second casing for accommodating therein said optical element andsaid first casing; a first supplier, having a first port in said firstcasing and a second port in said second casing, for supplying an inertgas into said first casing and said second casing through said firstport and said second port; and a second supplier, having a third port insaid first casing and a fourth port in said second casing, for supplyinga gas, containing oxygen, into said first casing and said second casingthrough said third port and said fourth port.
 23. An apparatus accordingto claim 22, wherein said first port functions also as said third port,and said second port functions also as said fourth port.
 24. Anapparatus according to claim 22, further comprising (i) a fifth portprovided in said first casing, (ii) a sixth port provided in said secondcasing, and (iii) an exhauster for discharging a gas in said firstcasing and said second casing through said fifth port and said sixthport.
 25. A method of manufacturing a device, said method comprisingsteps of: exposing a substrate to a pattern of a mask by use of anexposure apparatus as recited in claim 22; developing the exposedsubstrate; and processing the developed substrate to produce the device.